1. Field of the Invention
The present invention relates to polyolefin blends and in particular to a physical admixture of a major portion of particles of an ethylene polymer with a minor portion of particles of a composition of an ethylene polymer and an agent that is capable of reacting with polyolefins in the molten state, such reactive agent being a cross-linking agent and/or a modifying agent and being further defined hereinbelow.
2. Description of the Prior Art
Polymers of ethylene, for example, homopolymers of ethylene and copolymers of ethylene and higher alphaolefins, are used in large volumes for a variety of end-uses, for example, in the form of film, fibres, moulded or thermoformed articles, pipe, coatings and the like.
Polyolefin compositions as offered for sale and/or used in such end-uses often contain reactive and/or non-reactive agents to modify or stabilize the polymer during processing or during use of articles fabricated from the compositions. Typical reactive agents include cross-linking agents and certain unsaturated compounds. Typical non-reactive agents include antioxidants and other stabilizers, nucleation agents and additives that affect the slip or blocking characteristics of products or the release of products from moulds used in fabrication processes. It is important that the incorporation of agents into polyolefin compositions be carried out so that the resultant composition has uniform properties.
Polymers having properties that are commercially-acceptable in a variety of end-uses are known. In addition, improvements in some polymer properties could lead to improved products and/or use of the polymers in additional end-uses. For instance, one method of improving the end-use characteristics of an article rotationally moulded from polymers of ethylene is to incorporate a cross-linking agent, for example, an organic peroxide, into the polymer composition. In the rotational moulding of a composition containing an organic peroxide, the polymer flows to coat the inside of the mould and then the cross-linking agent causes crosslinking of the polymer so as to increase the molecular weight of the polymer, thereby improving end-use properties of the resultant article. Cross-linkable compositions especially adapted for rotational moulding end-uses are disclosed in European Patent Publication No. 0 087 210 of G. White, published 1983 Aug. 31.
Cross-linkable compositions may also be used in other end-uses to obtain improvements in product properties; the cross-linking of the polymer will tend to affect melt characteristics of the polymer under low shear rate processing conditions. One example of the use of partial cross-linking to obtain an improvement in film properties is disclosed in Canadian Patent No. 1 123 560 of D. A. Harbourne, which issued 1982 May 18.
Techniques for the incorporation of agents into polyolefins are well known in the art. Non-reactive agents, especially stabilizing agents, are frequently incorporated into molten polymer during the process for the manufacture of the polymer. Reactive and non-reactive agents may also be incorporated into polymers by melt blending techniques in which the agent is metered into or otherwise added to molten polymer during extrusion of the polymer into pellets or a fabricated article.
It is important in the addition of agents that the agent be uniformly distributed throughout the polymer. With non-reactive agents, however, the uniformity of the distribution of the agent is normally less critical than with reactive agents. For example, the requirements for a slip agent or a stabilizer may be less critical than for a cross-linking agent. The cross-linking of a polymer increases the molecular weight of the polymer. Thus, in order to obtain a product of uniform properties, especially properties dependent on molecular weight, it is important that the cross-linking of the polymer be carried out in a uniform manner. If the cross-linking is not uniform, the resultant product may, for example, have areas of weakness due to either insufficient or excessive cross-linking of the polymer or have gel particles resulting from excessive cross-linking of the polymer, such gel particles resulting in unacceptable visual appearance and/or areas of weakness in the product. Some fabrication processes, for example blow moulding of bottles and the manufacture of sheet and film, may be more sensitive to non-uniform product properties than other processes. In order to obtain uniform product properties, it has been necessary to use expensive and/or complex processes to incorporate reactive agents into polymers, including special handling facilities and special extruder screw designs.
The blending of organic peroxide with molten polyethylene is disclosed in U.S. Pat. No. 3,182,033 of R. S. Gregorian, which issued 1965 May 04, and in Canadian Patent No. 957 473 of H. J. Cook, which issued 1974 Nov. 12. D. A. Alia disclosed in U.S. Pat. No. 4,197,381, which issued 1980 Apr. 08, that the blending of crystalline polymers and amorphous elastomeric polymers aided in the production of cross-linkable polymers having a homogeneous composition. The blending of reactive agents into polyethylene by means of a physical blend of polyethylene with a different polymer containing the reactive agent is disclosed in the application of D. W. Boivin and R. A. Zelonka, Ser. No. 756,223 filed 1985 July 18.